The present invention relates generally to rolling metal products and particularly to providing such products with an anisotropic engineered surface texture that provides improved uniform brightness.
A surface appears bright to the human eye when the surface reflects incident light specularly, i.e., when the light striking the surface is not significantly diffused. Specular reflection, in turn, requires a non-random surface finish so that light is reflected from the surface at the same angle it was incident to the surface (which is the definition of specular reflection). A random surface diffuses incident light and thus makes the surface appear dull to the human eye, i.e., incident light is reflected randomly in many directions because of the random orientations of surface roughness; the internal order of the incident light is hence not preserved.
In providing a rolled sheet product with a bright surface, the surface of the work roll employed to produce the product must also have a topography that is engineered to provide a high degree of regularity. Traditional methods of finishing work rolls involve one or more grinding operations. Grinding, however, does not provide roll surfaces with uniform textures since grinding is very much a stochastic process which results in a ground texture height, measured from an average datum line from which average roughness can be measured, that follows a normal or Gaussian distribution. The distribution of roughness is influenced by the abrasive particle size in the grinding medium (wheel), the feed rate of the roll in relation to the grinding medium, depth of cut and the number of grinding passes.
In manufacturing aluminum can end stock, for example, the customer desires the stock (sheet) to have a uniformly bright, highly reflective surface, with a certain composite surface roughness that is smooth to the human touch and appears shiny to the human eye. This requires the rolling operation to be conducted in the boundary lubrication regime, which means that there is significant metal-to-metal contact. The texture of the roll surface may then be faithfully imprinted onto the sheet surface.
With present state-of-the-art roll grinding, the rolling of aluminum sheet in the boundary lubrication regime to create a bright surface at high speeds (e.g. 4000 ft. per minute) is difficult with relatively large (typically 22 inch diameter) work rolls. There are three primary reasons for this: (1) the grinding process generates variable depth grooves, i.e., the depths of two successive grooves may be quite different in the roll surface, which results (locally) in partial or total separation of the roll surface from the sheet surface due to the generation of thick lubricant films, (2) a ground roll surface produces a non-uniform texture height on the sheet surface due to the Gaussian distribution of surface roughness, as discussed above, resulting in diffuse reflection of light, and (3) a ground roll surface has non-uniform wear characteristics, which result in inconsistencies in the rolling operation, i.e., rolling speed must be changed (lowered) to accommodate the worst case condition on the roll surface. (Ground rolls, in addition, require frequent regrinding, which adds cost to the rolling process.) It is well known that the thickness of a lubricating film is a function of the square root of roll diameter such that larger work rolls are more of a problem than smaller work rolls. In reference to rolling speed, film thickness is a linear function of velocity.
As explained earlier, a bright, highly specularly reflective surface is one that reflects light primarily at the angle at which the light strikes the surface, i.e. the angle of incidence, rather than reflecting the light in a diffuse manner. The ratio of diffuse to specular reflection, which is the amount of reflected light measured at the angle of incidence compared to the amount of light measured at two degrees from incidence, is a good measure of surface brightness. The lower this ratio the greater is the surface brightness.
Diffuse reflection may also occur in the presence of micro-size cracks or fissures. Fissures are generally created when a product is rolled under hydrodynamic lubricating conditions which means that roll and product surfaces are either locally or entirely separated by a lubricant film. This is especially true for the high speeds at which aluminum sheet is rolled. If fissures pre-exist in the product surface, they may be enhanced since the hydrodynamic pressure in the lubricant film forces lubricant into such cracks to widen and deepen them. Fissures generally extend in a direction that is transverse to the direction of rolling, and can occur in both steel and aluminum products.
The result, then, of a ground roll surface is a random, stochastic texture imparted to a rolled product's surface, including fissures, such that the surface appears dull to the human eye.